Enhanced HONO Formation from Aqueous Nitrate Photochemistry in the Presence of Marine Relevant Organics: Impact of Marine-Dissolved Organic Matter (m-DOM) Concentration on HONO Yields and Potential Synergistic Effects of Compounds within m-DOM

Nitrous acid (HONO) is a key molecule in the reactive nitrogen cycle. However, sources and sinks for HONO are not fully understood. Particulate nitrate photochemistry has been suggested to play a role in the formation of HONO in the marine boundary layer (MBL). Here we investigate the impact of marine relevant organic compounds on HONO formation from aqueous nitrate photochemistry. In particular, steady-state, gas-phase HONO yields were measured from irradiated nitrate solutions at low pH containing marine-dissolved organic matter (m-DOM). m-DOM induces a nonlinear increase in HONO yield across all concentrations compared to that for pure nitrate solutions, with rates of HONO formation increasing by up to 3-fold when m-DOM is present. Furthermore, to understand the potential synergistic effects that may occur within complex samples such as m-DOM, mixtures of chromophoric (light-absorbing) and aliphatic (non-light-absorbing) molecular proxies were utilized. In particular, mixtures of 4-benzoylbenzoic acid (4-BBA) and ethylene glycol (EG) in acidic aqueous solutions containing nitrate showed more HONO upon irradiation compared to solutions containing only one of the molecular proxies. This suggests that synergistic effects in the HONO formation can occur in complex organic samples. Atmospheric implications of the results presented here are discussed.


Data Analysis
The DOASIS analysis software for differential optical spectroscopy was used to deconvolute the data acquired by the IBBCEAS. 1 The literature HONO and NO2 absorption cross sections were convoluted to match the resolution of the detector used. 2,3The software deconvolutes the acquired spectra by fitting the data to these literature values.The rest of the data is fit to a polynomial containing non-HONO absorption and Mie and Rayleigh scattering or a residual trace for data not fitted to the reference spectra or the polynomial.This is shown in Figure S1 for a solution of 100 mM NaNO3 at pH 2.00.

Figure S1.
The experimental spectrum (black trace) is fit by DOASIS (red trace) and then separated into the absorbance due to HONO (blue trace) and NO2 (green trace) by fitting them to literature HONO and NO2 spectra.Also displayed are the absorbance due to Mie and Rayleigh scattering and any other non-HONO and NO2 absorptions fitted to a polynomial (purple trace) and any wavelength dependent residual absorption data not able to be fitted (gray trace).This is the deconvolution of one spectrum for an experiment of 100 mM NaNO3 at pH 2.

UV-Vis Absorption Spectroscopy of Molecular Proxies
The absorbance spectra of the molecular proxies used in this study was taken and compared to m-DOM.The range of absorption was 280-500 nm as this is the range of absorption of m-DOM that overlaps with the solar spectrum. 4,5Figure S2 displays 0.03 mg/mL m-DOM in dark blue having a broad absorption profile in the entire range.The absorbance from m-DOM in this region corresponds to the  →  * transition corresponding to the aromatic groups in m-DOM. 4The absorption of 0.44 mM 4-BBA (pink) has a peak not shown in Figure S3 at 260 nm, but the tail of this absorption is shown and correspond to the both the  →  * and  →  * transitions. 6Lastly, 0.44mM EG (light blue) does not absorb light in this region, having some absorption starting at energies higher than 280 nm and peaking at 200 nm, which is higher energy than relevant in this study, making it a viable non-light absorbing marine boundary layer aliphatic proxy.

HONO Formation
The maximum, steady-state concentration observed for HONO, [HONO]max, in experiments with varying surface tension.Similarly, the relative rate constant was from Table 2 was plotted with respect of the concentration of m-DOM.•[HONO]max

HONO Profiles of Irradiated Nitrate Solutions Containing m-DOM
The HONO formation profiles for all solutions containing m-DOM are shown in Figure 3 and the decay profiles for solutions containing only nitrate and nitrate plus 0.10 mg/mL m-DOM are shown in Figure 5 of the main text.A control experiment was performed where solutions containing 100 mM NaNO3 plus 0.10 mg/mL m-DOM was exposed to light from the Xe-Arc lamp solar simulator for three hours and the decay was measured by only turning off the lamp.This same experiment was done where the lamp was turned off as well as the path of the carrier gas from the reaction cell into the IBBCEAS was cutoff while keeping the rate of carrier gas the same.
Figure S5 below shows that evacuation of HONO from the cavity happens within 30 minutes of cutting off the path from the reaction cell with the majority happening within the first 15 minutes.
In contrast, keeping the carrier gas flowing over the reaction cell leads to a HONO decay profile that requires more than 5 hours to no longer detect HONO.Therefore, the profiles observed are indicative of kinetics of the gases partitioning into the gas and not HONO adsorbing to the walls of the experimental setup.

Extraction of m-DOM from SeaSCAPE campaign
The extraction of m-DOM during the 2019-NSF CAICE SeaSCAPE Campaign has been previously described by Alves et al., 2022. 5The collaborative mesocosm project SeaSCAPE was described in detail by Sauer et al, 2021. 11 Briefly, approximately 11,800 L of seawater was gathered from Ellen Browning Scripps Memorial Pier in La Jolla, California.This seawater was filtered to 50 microns to eliminate non-microbial biota and large grazers.The filtered seawater was then transferred to a cleaned glass wave channel and allowed to stabilize at ambient temperature for 24 hours.Following this, nutrients were introduced into the wave channel with fluorescent lights to replicate a diurnal light cycle.Over the 23-day period of microbial bloom development, samples were collected, from which dissolved organic matter (DOM) was isolated through sequential filtration.Post-filtration, the inorganic carbon was removed by acidification to pH 2.00 using 1M HPLC-grade HCl.The resulting DOM was then purified via solid-phase extraction.
Specifically for this study, the DOM used was the DOM acquired from filtering the entirety of the seawater used at the end of the campaign.

Figure S2 .
Figure S2.The absorption of the molecular proxies 4-BBA and EG both at 0.44 mM in concentration.These are compared to 0.03 mg/mL of m-DOM all solutions contain only the organics in MQ water acidified to pH 2.0 with HCl.The shading corresponds to the error in the form of one standard deviation from triplicate measurements.In some cases the shading is not evident as the error is smaller than the width of the line.

Figure S3 .
Figure S3.The NO2 concentration measured in ppb during the irradiation period of the experiments (t = 1 -4 hours) for all solutions containing acidified 100 mM NaNO3 with varying amounts of m-DOM.The dashed traces correspond to the solutions that led to less HONO enhancement with increasing m-DOM concentration.

Figure S4 .
Figure S4.(A) Maximum concentration of HONO in ppb ([HONO]max) measured from irradiating aqueous nitrate solutions as a function of surface tension.The error bars represent one standard deviation.(B) Relative formation rates of HONO from solutions of nitrate and m-DOM with different m-DOM concentrations to HONO from solutions of with nitrate only.

Figure S5 .
Figure S5.The decay (royal blue) or evacuation (cyan) of HONO was measured in the experimental set up.Time = 0 is the point where the acidified to pH 2.0 solutions of 100 mM NaNO3 + 0.10 mg/mL m-DOM where no longer exposed to the solar simulator.

Figure S6 .
Figure S6.The NO2 concentration measured in ppb during the irradiation period of the experiments (t = 1 -4 hours) for all solutions containing acidified 100 mM NaNO3 with mixtures of the molecular proxies 4-BBA and EG, the molecular proxies on their own, and 0.03 mg/mL m-DOM.